1. Field of the Invention
The present invention relates to a dynamoelectric machine and, more particularly, to a dynamoelectric machine of the type in which a magnetic field is generated by a magnetic field generating coil supplied with an electrical current.
2. Description of the Related Art
FIG. 1 is a sectional view of a conventional dynamoelectric machine, e.g., an electric motor, and FIG. 2 is a sectional view of a stator of the machine shown in FIG. 1. Referring to these figures, the dynamoelectric machine has a casing composed of a motor frame 1, a loadside bracket 2 and another bracket 3 opposite to the loadside bracket 2. The brackets 2 and 3 respectively support bearings 4 and 5. The machine also has a stator core 6 which has a laminated structure composed of a plurality of stator core plates 6a (see FIG. 3). Numeral 7 denotes coil ends which are ends of a magnetic field generating coil 11 which is wound through later-mentioned slots (see FIG. 4) of the stator core 6. A rotor 8 is composed of an armature core and an aluminum die cast member. A motor shaft 9 integral with the rotor 8 is rotatably supported by the bearings 4 and 5. An insulator 10 electrically insulates the stator core 6 from the magnetic field generating coil 11 (including the coil ends 7).
FIGS. 3(a) is a plan view of the single element of the stator core plate 6a of the stator core 6 before the lamination, while FIG. 3(b) is a side view of the stator core plate 6a. FIG. 4 is an enlarged view of a slot portion 12 which receives a magnetic field generating coil 11. A throttle cell 13 for providing electrical insulation between the magnetic field generating coil 11 and the stator core 6 is provided around the magnetic field generating coil 11. Slot wedges 14 are provided for preventing the magnetic field generating coil 11 from coming out of the slot 12.
FIGS. 5 and 6 show the arrangement of coils which is adopted when the stator core 6 has 12 slots to realize a 3-phase 4-pole construction, as well as magnetomotive forces of the respective phases and the composite megnetomotive force produced by all the phases. FIG. 7 shows an example of the coil arrangement of single-phase 4-pole stator core with 24 slots.
The operation of this known machine is as follows.
A description will be given first of the process of producing the stator core, as well as the principle of generation of the magnetomotive force which produces the torque for driving the machine which is, in this case, a motor. Stator core plates 6a of the type shown in FIG. 3 are formed as the first step of the production process. When the motor is a 3-phase motor, the following condition is generally met. EQU Ns=q/(p.times.3)
where, Ns represents the total number of the slots in the stator core plate, q represents the number of slots for each pole in each phase, and p represents the number of poles.
The greater the number q of slots for each pole in each phase, the higher the motor performance. An excessively large total number Ns of the slots, however, causes the circumferential length occupied by the slot widths b to be increased, so that the slot intersal l is decreased excessively, with the result that the strength of the stator core is decreased impractically.
In addition, insufficient strength of the slot interval portions l tends to cause a problem in that the edges of the slots 12 of the stacked stator core plates 6a in the laminated stator core 6 are not registered, making it impossible to insert the coils into the slots 12. Usually, the slot interval l is 2.5 mm or so at the smallest.
In this case, the following condition is met: EQU .pi.D/Ns-=b
where, D represents the diameter of the portion of the stator core plate where the slots are formed (see FIG. 3(a)).
Therefore, the total slot number Ns can be written as follows. EQU Ns=.pi.D/(b+l)
The total number Ns of the slots can be increased by decreasing the slot width b. A reduction in the slot width b, however, produces the following problem. Namely, the proportion of the space in each slot 12 occupied by the slot cell 13 and the slot wedge 14 serving as an insulator is increased to correspondingly decrease the space factor, i.e., the portion of the space in each slot which is to be occupied by the magnetic field generating coil 11. In the worst case, it becomes impossible to insert the magnetic field generating coil into the slot 12.
For these reasons, ordinary small-sized 4-pole motors have 12, 24 or 36 slots.
Referring back to the production process, a plurality of stator core plates 6a shown in FIG. 3 are stacked to form a laminated stator core 6. Then, slot cells 13 (see FIG. 4) are inserted into the respective slots 12, followed by insertion of the magnetic field generating coils 11 into these slots. Then, slot wedges 14 are fitted into the slots 12, whereby the stator is completed as shown in FIG. 2.
FIGS. 5 and 6 show the magnetomotive forces produced by the respective phases, as well as the composite magnetomotive force produced by all these phases, when the stator core 6 is of 3-phase 4-pole type. FIG. 5(A) shows the manner in which the field generating coil of the U phase, as the representative of three phases U, V and W, is inserted, while FIG. 5(B) shows the magnetomotive force of the coil on each pole of the U phase. In FIG. 6, (A), (B) and (C) respectively show the magnetomotive forces produced by the U, V and W phases, while FIG. 6(D) shows the composite magnetomotive force produced by the three phases. The characteristics shown in these figures are obtained on an assumption that when the current in the U phase is at the maximum value Im, the currents in the V and W phases are -Im/2. Since the magnetic field generating coils of the U, V and W phases have an equal number of turns, the waveform of the composite 3-phase magnetomotive force does not change in relation to the elapse of time, although the composite magnetic field moves in the direction of rotation.
FIG. 7(A) shows the manner of insertion of the main coils of the magnetic field generating coil in a stator core of of single-phase 4-pole type with 24 slots. FIGS. 7(B) and 7(C) show the magnetomotive forces produced by the main coils a and b. FIG. 7(D) shows the composite magnetomotive force produced by these main coils. When the current in the level of the current in the main coil is changed, the height of the waveform of the composite magnetic motive force is changed but no change is caused in the waveform. In case of the single phase motor, the numbers of turns of the main coils a and b may be different from each other. The characteristic shown in FIG. 7, however, is obtained when the main coils a and b have an equal number of turns. Any difference in the number of turns between the main coils a and b only causes a change in the levels of the magnetomotive forces.
In the stator of the conventional dynamoelectric machine, it it not permitted to limitlessly increase the total number of slots. Namely, the total number of the slots must be a small definite number. This makes it impossible to generate a magnetic field of sine waveform. On the other hand, an increase in the total number of slots requires an increase in the diameter of the stator core, so that the efficiency of the motor is decreased due to generation of extraordinary torque and high-frequency loss. In addition, the temperature of the motor is raised undesirably.